The electrospray process consists of flowing sample liquid through a small tube or capillary which is maintained at a high voltage with respect to a nearby surface. The liquid is dispersed into fine electrically charged droplets or by the voltage gradient of the tip of the capillary. The ionization mechanism involves the desorption at atmospheric pressure of ions from the fine electrically charged particles. In many cases, a heated gas is flowed in counter-current to the electrospray to enhance dissolution of the electrospray droplets. The ions created by the electrospray are then mass analyzed in a mass analyzer such as a mass spectrometer.
Under the appropriate conditions, the electrospray resembles a symmetrical cone consisting of a very fine mist (or fog) of droplets (ca. 1 .mu.m diameter). Excellent sensitivity and ion current stability can be obtained if the fine mist is produced. Unfortunately, the electrospray "quality" is highly dependent on the bulk properties of the solution being analyzed. The most important of which are surface tension and conductivity. A poor quality electrospray may contain larger droplets (&gt;10 .mu.m diameter) or a non-dispersed droplet stream.
The use of sheath liquid and a focusing gas are often used to insure stable sprays when electrospraying high aqueous content sample solutions. One type of electrospray interface apparatus includes an inner needle for transferring a liquid sample to an ionizing region at one end of the needle, a first outer tube surrounding and spaced from said needle for flowing a liquid past the tip of said needle, and a second outer tube surrounding the first tube to define a second cylindrical space for flowing a gas past the end of said first tube and needle to focus the electrospray.
In U.S. Pat. No. 4,542,293 there is described the use of a tube made of an electrical insulator for conducting ions in the ionizing electrospray region at atmospheric pressure and a low pressure region. A glass or quartz capillary is suitable. Ions and gas are caused to flow from the ionization region through the tube into the low pressure region where free jet expansion occurs. A conductive coating is formed on the ends of the insulating tube and a voltage is applied thereacross to accelerate ions which flow through the tube. A conducting skimmer is disposed adjacent the end of the tube and is maintained at a voltage which causes further acceleration of the ions through and into a lower pressure region including focusing lenses and analyzing apparatus.
The electrospray process employing a sheath liquid and focusing gas provides small droplets or particle which are desolvated by the addition of a counter current drying gas. Occasionally, larger undesolvated droplets or particles will traverse into the capillary and will acquire substantial kinetic energy which will allow it to pass through the skimmer and into the lens region. The droplets or particles impinge upon surfaces and forms secondary ions. These ions are random and cause noise to be observed at the analyzer detector, thereby decreasing the signal to noise levels and producing noise spikes in the ion chromatogram.